This is clearly demonstrated in the case of Cdk4/Cdk6, two essential cell cycle genes, that when deleted in combination (double KO) result in death at late stages of embryonic development (Malumbres et al. 2004), and yet Cdk4/Cdk6 dual inhibition has become a standard of care for many breast cancer patients, and preclinical data suggest it will also be the case for many other tumor types (Goel et al. 2022; Salvador-Barbero et al. 2020). This evidence concerns the gene CDK4 and breast cancer.